Pipe a is a inlet pipe filling the tank at 8000l/hr and pipe b is the outlet pipe which empties the tank in 3 hours. the capacity of the tank is

We let x as the time it takes to fill the tank. We calculate as follows:

1/x - 1/3 = 1
1/x = 1 + 1/3
1/x = 4/3
x = 3/4 = 0.75

Capacity = 8000 L/hr (0.75 hr) = 6000 L